Indiscriminate oophagy by ant larvae: an explanation for brood serial organization?

Abstract

Last instar larvae ofLasius niger under standard laboratory conditions and abundant food supply feed on conspecific eggs and, if forced to a choice, show a significant statistical preference for trophic versus fertilized eggs (approx. 60% of the trials observed). On the other hand, they are unable to discriminate between kin and non-kin eggs, both fertilized and trophic. Fertilized eggs killed by freezing and trophic eggs handled in the same way are also selected in a random manner. Last instar larvae ofMessor semirufus regularly failed to discriminate in a significant way between trophic and fertilized and between kin and non-kin eggs. Substantial experimental evidence confirms that larvae of both these species — as already suggested in the literature for other ants-rely on trophic eggs as an essential component of their diet in order to be able to develop. Additionally,L. niger last instar larvae appear to be able to attack and pierce the egg chorion with their own mandibles, while this capacity is at least very reduced or very rare inM. semirufus. First and second instar larvae of both species never succeeded in piercing the egg membrane alone. These findings imply that the “right” (i.e. trophic) eggs should be presented to the larvae by the workers (i.e. worker discrimination should be assumed) and, at least in the case of youngLasius larvae, and probably for all stages ofMessor larvae, the egg membrane must be pierced by the workers in order to allow the larvae to feed. The following evolutionary sequence is suggested to explain the origin of trophic eggs in ants: 1) larval oophagy (obligatory at least for the species founding new colonies in an independent claustral manner), 2) generalized facultative or obligatory larval oophagy for larvae of all colonial stages, 3) production of trophic eggs by the queen(s) and/or workers in order to avoid cannibalism of nestmates. Lack of discrimination or weak discrimination capacity between trophic and fertilized eggs and the consequent larval cannibalism in form of oophagy, as demonstrated in this paper, is suggested to explain age segregation among eggs and larvae from workers, a widespread phenomenon in ants which must have been selected to avoid the oophagy of viable eggs.